New research reveals a large asteroid
breakup to be the likely source of the impactor that caused a mass extinction
event on Earth 65 million years ago

Boulder, Colo. — Sept. 6, 2007 — The
impactor believed to have wiped out the dinosaurs and other life forms on Earth
some 65 million years ago has been traced back to a breakup event in the main
asteroid belt. A joint U.S.-Czech team from Southwest Research Institute® (SwRI®)
and Charles University in Prague suggests that the parent object of asteroid
(298) Baptistina disrupted when it was hit by another large asteroid, creating
numerous large fragments that would later create the Chicxulub crater on the
Yucatan Peninsula as well as the prominent Tycho crater found on the Moon.

The team of researchers, including Dr.
William Bottke (SwRI), Dr. David Vokrouhlicky (Charles University, Prague) and
Dr. David Nesvorny (SwRI), combined observations with several different
numerical simulations to investigate the Baptistina disruption event and its
aftermath. A particular focus of their work was how Baptistina fragments
affected the Earth and Moon.

At approximately 170 kilometers in
diameter and having characteristics similar to carbonaceous chondrite
meteorites, the Baptistina parent body resided in the innermost region of the
asteroid belt when it was hit by another asteroid estimated to be 60 kilometers
in diameter. This catastrophic impact produced what is now known as the
Baptistina asteroid family, a cluster of asteroid fragments with similar orbits.
According to the team's modeling work, this family originally included
approximately 300 bodies larger than 10 kilometers and 140,000 bodies larger
than 1 kilometer.

Once created, the newly formed
fragments’ orbits began to slowly evolve due to thermal forces produced when
they absorbed sunlight and re-radiated the energy away as heat. According to
Bottke, "By carefully modeling these effects and the distance traveled by
different-sized fragments from the location of the original collision, we
determined that the Baptistina breakup took place 160 million years ago, give or
take 20 million years."

The gradual spreading of the family
caused many fragments to drift into a nearby "dynamical superhighway" where they
could escape the main asteroid belt and be delivered to orbits that cross
Earth’s path. The team's computations suggest that about 20 percent of the
surviving multi-kilometer-sized fragments in the Baptistina family were lost in
this fashion, with about 2 percent of those objects going on to strike the
Earth, a pronounced increase in the number of large asteroids striking Earth.

Support for these conclusions comes from
the impact history of the Earth and Moon, both of which show evidence of a
two-fold increase in the formation rate of large craters over the last 100 to
150 million years. As described by Nesvorny, "The Baptistina bombardment
produced a prolonged surge in the impact flux that peaked roughly 100 million
years ago. This matches up pretty well with what is known about the impact
record."

Bottke adds, "We are in the tail end of
this shower now. Our simulations suggest that about 20 percent of the
present-day, near-Earth asteroid population can be traced back to the Baptistina
family."

The team then investigated the origins
of the 180 kilometer diameter Chicxulub crater, which has been strongly linked
to the extinction of the dinosaurs 65 million years ago. Studies of sediment
samples and a meteorite from this time period indicate that the Chicxulub
impactor had a carbonaceous chondrite composition much like the well-known
primitive meteorite Murchison. This composition is enough to rule out many
potential impactors but not those from the Baptistina family. Using this
information in their simulations, the team found a 90 percent probability that
the object that formed the Chicxulub crater was a refugee from the Baptistina
family.

These simulations also showed there was
a 70 percent probability that the lunar crater Tycho, an 85 kilometer crater
that formed 108 million years ago, was also produced by a large Baptistina
fragment. Tycho is notable for its large size, young age and its prominent rays
that extend as far as 1,500 kilometers across the Moon. Vokrouhlicky says, "The
probability is smaller than in the case of the Chicxulub crater because nothing
is yet known about the nature of the Tycho impactor."

This study demonstrates that the
collisional and dynamical evolution of the main asteroid belt may have significant
implications for understanding the geological and biological history of Earth.

As Bottke says, "It is likely that more
breakup events in the asteroid belt are connected in some fashion to events on
the Earth, Moon and other planets. The hunt is on!"

The article, "An asteroid breakup 160
Myr ago as the probable source of the K/T impactor," was published in the Sept.
6 issue of Nature.

The NASA Origins of Solar Systems,
Planetary Geology and Geophysics, and Near-Earth Objects Observations programs
funded Bottke's and Nesvorny's research; Vokrouhlicky was funded by the Grant
Agency of the Czech Republic.